1. Field of the Invention
The present invention relates to the production of cellulolytic and/or hemicellulolytic enzymes, in particular in the context of the production of ethanol from cellulosic or ligno-cellulosic materials.
2. Description of Related Art
Since the 1970s, the transformation of ligno-cellulosic biomass to ethanol after hydrolysis of the constituent polysaccharides to fermentable sugars has formed the basis of a great deal of work.
Deciduous tree wood and cereal straw are the most widely used substrates. They are mainly constituted by about 40% to 50% of cellulose, 20% to 25% of hemi-cellulose and 15% to 25% of lignin.
Other resources, dedicated forest cultures, residues from alcohologenic plants, sugar refineries and cereal processors, residues from the paper industry and transformation products from cellulosic or ligno-cellulosic materials can be used.
Of those three polymers, cellulose, hemi-cellulose and lignin, cellulose is the principal source of fermentable sugars for fermenting to ethanol as it is constituted by glucose, which is readily fermented to ethanol by Saccharomyces cerevisiae in proven, high performance industrial processes. The pentoses contained in hemi-celluloses are not efficiently converted into ethanol. Other microorganisms form the genii Saccharomyces, Pichia, Candida, Pachysolen, Zymomonas, Klebsiella, Escherichia, may be selected to upgrade the monomeric sugars derived from the biomass to ethanol.
The process for transforming ligno-cellulosic materials to ethanol (see FIG. 1) comprises a physico-chemical pre-treatment step followed by an enzymatic or chemical hydrolysis step, a step for ethanolic fermentation of the sugars released and a step for recovering ethanol.
The pre-treatment step is aimed at liberating the sugars contained in the hemi-celluloses in the form of monomers, essentially pentoses, such as xylose and arabinose, and hexoses, such as galactose, mannose and glucose, and to improve the accessibility of the cellulose gummed into the lignin and hemi-cellulose matrix. A number of techniques exist: acid boiling, alkaline boiling, steam explosion, organosolv processes, etc. The pre-treatment efficacy is measured by the degree of hemi-cellulose recovery and by the susceptibility of the cellulosic residue to hydrolysis. Mild acid pre-treatments and steam explosion are the best techniques. They allow complete recovery of pentoses and good accessibility of the cellulose to hydrolysis.
The cellulosic residue is hydrolyzed, either by the acid method or by the enzymatic method using cellulolytic and/or hemicellulolytic enzymes. Microorganisms such as fungi from the genii Trichoderma, Aspergillus, Penicillium or Schizophyllum, or anaerobic bacteria, for example from the genus Clostridium, produce such enzymes, contain mainly cellulases and xylanases, suitable for complete hydrolysis of polymers constituting the plants.
The acidic method, carried out with a strong acid, in particular sulphuric acid, is effective but requires large quantities of chemicals (acid then base for neutralization). Enzymatic hydrolysis does not suffer from that disadvantage; it is carried out under mild conditions and is effective. In contrast, the cost of enzymes is still very high. For this reason, a great deal of work has been carried out to reduce the cost: increasing the production of enzymes in the first place, by selecting hyperproductive strains and by improving the fermentation conditions, reducing the quantity of enzymes in the hydrolysis, and in the second place, by optimizing the pre-treatment phase or by improving the specific activity of said enzymes. During the last decade, most studies have concerned understanding the mechanisms of the action of cellulases and the expression of enzymes to cause the enzymatic complex which is the most appropriate for hydrolysis of the ligno-cellulosic substrates to be excreted by modifying the strains with molecular biology tools.
The most commonly used microorganism for the production of cellulases is the fungus Trichoderma reesei. Wild type strains are able to excrete, in the presence of an inducing substrate, for example cellulose, the enzymatic complex considered to be the best adapted to cellulose hydrolysis. The enzymes of the enzymatic complex contain three major types of activities: endoglucanases, exoglucanases and cellobiases. Other proteins with properties which are vital to the hydrolysis of ligno-cellulolytic materials are also produced by Trichoderma reesei, for example xylanases. The presence of an inducer substrate is vital to the expression of cellulolytic and/or hemicellulolytic enzymes. The nature of the carbon-containing substrate has a large influence on the composition of the enzymatic complex. This is the case with xylose which, associated with a carbon-containing inducer substrate such as cellulose or lactose, can significantly improve xylanase activity.
Conventional genetic mutation techniques have enabled strains of Trichoderma reesei which are hyperproductive in cellulases to be produced, such as the MCG77 (Gallo, U.S. Pat. No. 4,275,167 A), MCG 80 (Allen A L and Andreotti R E, Biotechnol-Bioeng 1982, 12, 451-459, 1982), RUT C30 (Montenecourt, B S and Eveleigh D E, Appl Environ Microbiol 1977, 34, 777-782) and CL847 (Durand et al, 1984, Proc Colloque SFM, “Génétique des microorganismes industriels”, Paris, H HESLOT Ed, pp 39-50). The improvements have produced hyperproductive strains which are less sensitive to the catabolic repression on monomer sugars, for example glucose, compared with wild type strains.
Recombinant strains have also been obtained from strains of Qm9414, RutC30, CL847 Trichoderma reesei by cloning heterologous genes, for example Aspergillus niger invertase, to diversify the source of carbon necessary for the production of cellulases, and/or over-express cellobiase to improve the enzymatic hydrolysis yield, cellobiases being considered to be limiting enzymes in the reaction. Said strains have conserved their hyperproductivity and aptitude for cultivation in the fermenter.
The process for producing cellulases by Trichoderma reesei has formed the subject matter of major improvements with a view to extrapolation to the industrial scale.
To obtain good enzyme productivity, it is necessary to add a source of carbon which can be rapidly assimilated to allow the Trichoderma reesei to grow and an inducer substrate which allows expression of cellulases and secretion into the culture medium. Cellulose may play both roles; however, it is difficult to use on an industrial scale and has been replaced by soluble sources of carbon, glucose, xylose or lactose, lactose also acting as an inducer substrate. Other soluble sugars such as cellobiose and sophorose have been described as inducers, but they are too expensive to be used on an industrial scale. However, it has been established that the production of cellulases by Trichoderma reesei with soluble substrates is far inferior to those obtained on cellulose in a batch process. This is due to the repressor effect of readily assimilatable sugars at high concentrations. Continuously supplying soluble carbon-containing substrates lifts catabolic repression by limiting the residual concentration in the cultures and optimizing the quantity of sugar, producing a better yield and better enzymatic productivity (see French Patent No.2 555 603 B).
In an industrial process for the production of cellulolytic enzymes, lactose remains one of the most suitable substrates and one of the cheapest; however, it is still expensive and represents about a third of the cost price of the enzymes. Despite all of the progress made, the cost of enzymes when transforming cellulosic biomass to ethanol remains high, at 30% to 50%; further, when using lactose as the carbon source for the production of cellulases, the process is dependent on an external source of carbon. For this reason, the use of carbon-containing substrates from spinning, for example hydrolyzed hemi-celluloses, is an important advance if the source of the inducing carbon is readily available.